\begin{fmffile}{3bos}
\begin{fmfgraph*}(100,100)
\fmfleft{i1,i2}
\fmfright{o1}
\fmfdot{v1}
\fmf{boson}{i1,v1}
\fmf{boson}{i2,v1}
\fmf{boson}{o1,v1}
\fmflabel{\small{$g$}}{v1}
\end{fmfgraph*}
\end{fmffile}
